One-way clutches, which transmit power in a fixed direction, are, for example, provided at right and left ends of a drive shaft of a self-propelled walk-behind lawn mower in order to facilitate turning of the lawn mower and allow the lawn mower, when moved with an engine stopped, to be easily pushed and moved by hand (see, e.g., U.S. Pat. No. 4,909,365 and Japanese Patent Laid-Open Publication No. 2001-59531).
First, a freewheel clutch disclosed in U.S. Pat. No. 4,909,365 will be described with reference to FIG. 9.
Referring to FIG. 9, a clutch mechanism 100 is used for transmitting a driving force of a drive shaft 101 to a pinion 102. The clutch mechanism 100 includes, as its main elements, a key 103 and a friction disc 104. The key 103 has a wedge portion 103a to be received in a first keyway 105 axially formed in the drive shaft 101, in such a manner as to be able to rock therein to extend therefrom and retract thereinto, and a projecting portion 103b to be received in a hollowed portion 104a formed in the friction disc 104. The wedge portion 103a has a thick side portion forming an engaging portion 103c. 
When the drive shaft 101 is rotated, the key 103 is simultaneously rotated, causing the projecting portion 103b of the key 103 to abut on the hollowed portion 104a of the friction disc 104. Since the friction disc 104 is in frictional engagement with a housing 106, the projecting portion 103b of the key 103 is not moved further. Thus, the wedge portion 103a of the key 103 is cocked, projecting from within the first keyway 105. The engaging portion 103c of the wedge portion 103a engages one of a plurality of second keyways 102a formed in the inner peripheral surface of the pinion 102, thereby ensuring transmission of a driving force of the drive shaft 101 to the pinion 102.
When the drive shaft 101 is not rotated, the wedge portion 103a of the key 103 is received within the first keyway 105 without being cocked, and the pinion 102 can rotate bidirectionally relative to the drive shaft 101.
The above clutch mechanism 100, however, has a problem that, for producing rotation resistance at the friction disc 104 by a thrust spring 107, it is necessary to reduce variations in friction coefficient between the surface of the housing 106, the opposite surfaces of the friction disc 104 and the surface of the pinion 102, resulting in time-consuming surface treatment of these components.
Second, a power transmission mechanism disclosed in Japanese Patent Laid-Open Publication No. 2001-59531 will be described with reference to FIGS. 10A to 10C.
A power transmission mechanism 200 shown in FIG. 10A includes a drive shaft 201, a plurality of pins 203 (three in the illustrated example) fitted in a plurality of cam grooves 202 formed in the drive shaft 201, a gear 204 and a friction disc 205 fitted onto the drive shaft 201, and an oil seal 206 fitted onto the friction disc 205. When the drive shaft 201 is not rotated, the pins 203 are detached from the inner peripheral surface of the gear 204 forming a hole 207, allowing the gear 204 to rotate bidirectionally relative to the drive shaft 201.
The above power transmission mechanism 200, however, requires highly accurate formation of the surfaces constituting the cam grooves 202, the outer peripheral surfaces of the pins 203 and the inner peripheral surface of the gear hole 207. Specifically, as shown in FIGS. 10B and 10C, it is necessary to set the dimensional tolerance of the cam grooves 202 and the pins 203 small. If not, excessive or insufficient contact can occur between the three pins 203 and the gear hole 207, causing the pins 203 to be likely to have serpentine behavior, sliding and suffering great impacts when transmitting power.
Thus, it is desired to ensure transmission of power without impacts by a simple configuration having no need to set the accuracy of fabricating each component at a high degree.